1. Field of the Invention
The present invention relates to an exhaust gas purifying catalyst deterioration detecting system for detecting the deterioration, if any, of a catalyst which is disposed in the exhaust system of an internal combustion engine for purifying the engine exhaust gas.
2. Related Art
In order to prevent a vehicular exhaust gas purifying system from being run with the catalyst deteriorated so as to have a lower exhaust gas purifying ability, a catalyst deterioration detecting system for detecting the deterioration of the catalyst (as disclosed in Japanese Patent Laid-Open Publication No. 2-136538 or 3-253714, for example). These catalyst deterioration detecting systems are made to detect the deterioration of the catalyst from a reduction in the exhaust gas purifying ability after the catalyst is warmed up to an activation temperature (generally 300 to 400.degree. C. or higher).
Before the catalyst is warmed up to the activation temperature after an engine start, even the normal catalyst has a low exhaust gas purifying ability; however, the deteriorated catalyst has a much lower exhaust gas purifying ability, and thus increases to increase noxious contents (or emissions) in the exhaust gas. According to both of the aforementioned catalyst deterioration detecting methods of the prior art, however, the deterioration of the catalyst is detected from the drop in the exhaust gas purifying ability after the activation of the catalyst, thereby making the catalyst deterioration detection difficult while considering the emission increasing degree before the catalyst activation. The increase in the emissions before the catalyst activation may erroneously determine that the catalyst which is to be intrinsically determined deteriorated is not deteriorated.
If the engine is started cold, the activation of the catalyst proceeds from the inflow (or upstream) side of the exhaust gas as the time elapses after the cold-start, until the entire catalyst is finally activated. Generally, the catalyst is given some surplus in its entire capacity so that it can achieve, even if deteriorated more or less once entirely activated, a purification factor approximate to that of a new one thereby to reduce the difference in the purification factor between the deteriorated and new catalysts. As the activation of the catalyst proceeds the better, it becomes more difficult to discriminate between (i.e., to detect the catalyst deterioration) the deteriorated and new catalysts. As a result, when the deterioration of the catalyst is detected from the drop in the exhaust gas purifying ability after the catalyst activation, the emission increase before the catalyst activation may determine no deterioration of the catalyst, which should be intrinsically determined to be deteriorated.